It is known to use self-propelled ram boring machines to lay lines without digging trenches. A pneumatically driven ram boring machine known from German Pat. No. 21 57 259 has a striking piston guided in the machine housing and a movable impact tip on which the automatic piston periodically strikes ramming blows. Under the influence of the ramming blows the impact tip moves oscillatingly into the ground and finally draws the housing after it when its stroke is exhausted. Machines of this kind are mainly used to lay supply lines, for example water pipes, electric cables or telephone lines, beneath roads or sidewalks without having to tear up the surface of the ground or sidewalk at the same time. When the ram boring machine moves through the ground it pushes the earth to the side and leaves a tunnel in the ground into which a supply line can be simultaneously or subsequently drawn. However, it is not easy to make a tunnel whose course can be directed. This depends on the composition of the ground, the type of line to be laid and other external influences.
A self-propelled pneumatic ram boring machine known from German Pat. No. 22 42 605 is provided with guiding wings. These are intended to prevent the ram boring machine from moving away from the desired direction; they serve to stabilize the direction and possibly also to guide the ram boring machine on a preselected curved path if the guiding wings are curved. Despite the guiding wings, and although during the forward movement the impact tip shatters or pushes to the side stones or other obstacles in the ground, i.e. drives a passage for the following machine, the possibility cannot be ruled out that the ram boring machine may move away from the desired direction when it strikes an obstacle. Directional deviations of this kind are of little importance in the case of short earth bores, such as for example when crossing under roads and railway embankments, but they do lead to considerable difficulties if accurately targeted and directed earth bores are to be made over long distances. In order to be able to control the direction of advance of the ram boring machine in the case of earth bores extending over long stretches target pits are therefore necessary at intervals.